Showing papers in "The American Naturalist in 1980"
TL;DR: Data on succession, productivity, and toughness indicate that E. menziesii shifts from an opportunistic strategy during its juvenile stages to a highly differentiated complex form able to persist in mature successional stages, thus implicating selection for persistence as opposed to rapid growth in climax communities.
Abstract: A synthetic "functional form" paradigm concerning hypothetically important adaptive features of algal structure and function was developed and tested by a costs/benefits strategic approach. Successional manipulations were performed by disturbing mature, environmentally constant intertidal communities; from the array of colonizing macroalgae, Ulva sp. was chosen as an opportunistic representative of pioneer seral stages, Egregia menziesii and Gelidium purpurascens/robustum as intermediate to late seral species, and Pelvetia fastigiata and Corallina officinalis as characteristic of more mature climax communities. The ranking from high to low primary producers (Ulva > Gelidium > Egregia > Pelvetia > Corallina) indicates that selection in fluctuating environments has favored opportunistic species having high net productivity, while those species able to persist in benign predictable habitats do so at the cost of lower photosynthetic rates and presumably slower growth. The kilocalories per ash-free gram dry we...
851Â citations
TL;DR: It is suggested that an 'optimum' thermal regime exists where adult size and fecundity are maximized; temperature regimes warmer or cooler than the "optimum'' result in small and less fecund adults.
Abstract: Adult body size and fecundity of several species of hemimetabolous aquatic insects were shown to depend largely on thermal conditions during larval growth We suggest that an "optimum" thermal regime exists where adult size and fecundity are maximized; temperature regimes warmer or cooler than the "optimum'' result in small and less fecund adults Two hypotheses concerning river water temperatures and size variation of adult insects are described First, maximum adult size reflects an equilibrium between several developmental processes that appear highly temperature dependent, viz, (i) the rate and duration of larval growth, and (ii) the specific time in larval development that adult structures begin maturing and the rate of this maturation process Second, a species distribution both locally within drainage systems and over a large geographic area is limited, in part, by lowered fecundity as adult size gradually diminishes in streams of increasingly cold or warm temperature cycles The importance of riv
787Â citations
TL;DR: This paper identifies the effects of different types of reproductive cost on the pattern of life histories, and argues that experimental tests of life history theory are not yet feasible, and that the authors must instead rely on comparative techniques.
Abstract: This paper identifies the effects of different types of reproductive cost on the pattern of life histories. By reproductive cost I mean the generally deleterious effect of present reproduction on f...
675Â citations
TL;DR: A graphical, equilibrium theory of resource competition allows prediction of the outcome of interactions between several consumers for the various classes of resources.
Abstract: The growth response of a population to the resources in a particular environment is used to classify pairs of resources as being either (1) essential, (2) hemi-essential, (3) complementary, (4) perfectly substitutable, (5) antagonistic, or (6) switching. Although nutrition is one important factor determining resource type, the growth response of a population to resources also depends on the interaction between a species' foraging methods and the spatial distribution of the resources. For example, two resources which are nutritionally perfectly substitutable may be operationally switching, antagonistic, or complementary because of spatial heterogeneity. A graphical, equilibrium theory of resource competition allows prediction of the outcome of interactions between several consumers for the various classes of resources. The technique requires information on (1) resource type (growth isoclines), (2) resource preference, (3) resource supply processes, and (4) mortality rates for all species. For all resource ...
627Â citations
TL;DR: At the ecosystem level, the most important consequence of the low energy requirements of amphibians and reptiles is their efficiency of biomass production, which greatly exceeds that of birds and mammals.
Abstract: The way of life of amphibians and reptiles, in contrast to that of birds and mammals, is based on low energy flow. Many of the morphological and physiological characteristics of ectothermal tetrapods that are normally considered to be primitive are in fact adaptations that facilitate a life of low energy demand. Their modest energy requirements allow amphibians and reptiles to exploit various adaptive zones unavailable to birds and mammals. Small body size is the most important of these; 80% of all lizard species and 90% of salamanders have adult body masses less than those of small birds and mammals. An elongate body form, a widespread and successful morphotype among amphibians and reptiles, is energetically unfeasible for endotherms. Amphibians and reptiles also are better suited than birds and mammals to ecological situations characterized by periodic shortages of food, water, or oxygen. At the ecosystem level, the most important consequence of the low energy requirements of amphibians and reptiles is ...
611Â citations
TL;DR: The model presented offers explanations of two ubiquitous patterns in nature: the canonical lognormal and the resulting species-area constant and an interesting invariance in the pattern of apportionment is observed for assemblages with three species.
Abstract: Recent proposals that the canonical lognormal distribution and the resulting species-area constant, $$z \simeq 1/4$$, are artifacts of the general lognormal curve and regression techniques, are sho...
520Â citations
TL;DR: It is concluded that Mayr's theory of allopatric speciation overemphasized both the genetic cohesion of widespread species and the founder effect on heterozygosity and quantitative genetic variation, and data on the strength of natural selection and the spontaneous mutability of quantitative characters provide a feasible microevolutionary mechanism for substantial and geologically rapid phenotypic evolution in small isolated populations.
Abstract: Quantitative genetic models of phenotypic evolution in small isolated populations are presented, from the initial founding event to continued random genetic drift and natural selection toward a new optimum phenotype. The basic features of complex morphologies included are polygenic inheritance with multiple allelism, pleiotropy, recombination and mutation. Gene flow, inbreeding depression, gene interaction, and genetic homeostasis are also discussed. Simpson's adaptive zones for phenotypes (analogous to Wright's adaptive topography for gene frequencies) are formulated probabilistically for small populations. It is concluded that Mayr's theory of allopatric speciation overemphasized both the genetic cohesion of widespread species and the founder effect on heterozygosity and quantitative genetic variation. However, data on the strength of natural selection and the spontaneous mutability of quantitative characters, in conjunction with the models, provide a feasible microevolutionary mechanism for substantial...
511Â citations
TL;DR: This study documents two primary sources of degradation of acoustic signals during propagation through natural environments, irregular amplitude fluctuations and reverberations, and finds intermediate frequencies (2-8 kHz) are most suitable for long-range acoustic communication.
Abstract: Effective communication requires that the receiver not only detect the presence of a signal but also discriminate significant variations in signals. Consequently, both attenuation and degradation o...
500Â citations
TL;DR: It can be concluded that it behooves all mammals to have as high a rate of metabolism as can be sustained by the quantity and quality of their food resources in space and time, because this adjustment will permit them to maximize their reproductive efforts.
Abstract: Data presented in this paper suggest that a complex interaction occurs between the physiological parameters of mammals and the growth and fluctuation of their populations: The Malthusian parameter rm increases with rate of metabolism, which in turn varies with body size and food habits. It can be concluded that it behooves all mammals to have as high a rate of metabolism as can be sustained by the quantity and quality of their food resources in space and time, because this adjustment will permit them to maximize their reproductive efforts. These interactions raise many questions, one of which concerns the temporal variation in reproductive strategies. Thus, Coady (1975) showed that the basal rate in the brown lemming (Lemmus trimucronatus) may be especially high during a winter in which the snow cover is reduced. Does a seasonal variation in the rate of metabolism therefore have a significant influence on seasonal variation in reproduction and therefore in population size? Whatever the correct answer to t...
478Â citations
TL;DR: It is suggested that females often largely determine the optimal male strategy which provides the optimal sperm displacement pattern for the females, and that selection on males independent of selection pressures on females is postulated to exert a major influence on the sperm precedence pattern of a population.
Abstract: It is postulated that the sperm precedence characteristics of most insect populations have resulted primarily from selection on females (1) to optimize the genetic composition of their progeny; (2) to discourage or encourage multiple matings for reasons other than genetic considerations; (3) to optimize their sperm storage capacity and utilization. In addition, selection on males to maximize egg fertilization by reduced displacement of their sperm and increased displacement of other sperm to the extent that these can be achieved without sacrificing the optimal mating strategy (Parker 1970a) probably has been important in some species. Parker's (1970a) hypothesis that the amount of sperm displacement in a population should stabilize at the value which yields the optimal overall male fertilization rate is accepted as the optimal male strategy; however, it is suggested that females often largely determine, by their behavior and by the structure and functioning of their reproductive tract, the optimal male st...
422Â citations
TL;DR: Presentation and dispersal of bird-disseminated fruits of woody plants in the eastern deciduous forest in terms of available dispersal agents is examined and a graphical model for disruptive selection in high- and low-quality fruits is presented.
Abstract: I have examined presentation and dispersal of bird-disseminated fruits of woody plants in the eastern deciduous forest in terms of available dispersal agents. Timing of fruit presentation and latit...
TL;DR: An energy-time budget model is developed which predicts the influence of various environmental factors upon feeding territory size and suggests that territory size should vary inversely with food production, but directly with competitor density for food-energy maximizers.
Abstract: An energy-time budget model is developed which predicts the influence of various environmental factors upon feeding territory size. For nonbreeding animals maintaining noncontiguous territories, territory size should (1) vary inversely with food production, but directly with competitor density, for feeding-time minimizers (defined here as animals that exhibit relatively fixed daily energy requirements); and (2) vary inversely with both food production and competitor density for food-energy maximizers, animals whose potential reproductive success is positively correlated with their net energetic intake. Concomitant predicted changes in time budgeting provide operational criteria for testing the model. Besides the primary effects of food and competitors, other factors may also influence territory size. Any competitors which successfully invade the territory can decrease the availability of food, forcing both time minimizers and energy maximizers to expand their territories. Where territories are contiguous,...
TL;DR: Theories of population genetics and evolutionary ecology: an introduction and some notes on measurement of the competition matrix are presented.
Abstract: Goh, B. S. 1977. Global stability in many species systems. Am. Nat. 111:135-143. MacArthur, R. H. 1969. Species packing, or what competition minimizes. Proc. Natl. Acad. Sci. USA 64:1369-1375. 1970. Species packing and competitive equilibrium for many species. Theor. Popul. Biol. 1:1-11. May, R. M. 1973. Stability and complexity in model ecosystems. Princeton University Press, Princeton, N.J. 1975. Some notes on measurement of the competition matrix. Ecology 56:737-741. Richardson, R. H., and P. E. Smouse. 1975. Ecological specialization of Hawaiian Drosophila. II. The community matrix, ecological complementation, and phyletic species packing. Oecologia 22: 1-13. Roughgarden, J. 1979. Theory of population genetics and evolutionary ecology: an introduction. MacMillan, New York.
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TL;DR: Causal mechanisms include the correlation of island area with habitat diversity, and sampling artifact if the number of samples is correlated with island size, as well as the immigration-extinction equilibrium theory (see Connor and McCoy 1979 for review of these hypotheses).
Abstract: Island biogeography theory predicts that the equilibrium number of species on an island, the net number of species resulting from immigration less extinction, is related to the area of the island and its distance from a source region (MacArthur and Wilson 1963, 1967). In numerous studies the relationship between species number, S, and island area, A, has been well described by the power function S = cAz, where c and z are fitted constants. May (1975) has shown that such speciesarea curves are to be expected if the relative abundance of species assumes a lognormal distribution. In such cases z values should lie within the range 0.150.39. As often noted, the species-area relation is an empirical description. Causal mechanisms include the correlation of island area with habitat diversity, and sampling artifact if the number of samples is correlated with island size, as well as the immigration-extinction equilibrium theory (see Connor and McCoy 1979 for review of these hypotheses). All three hypotheses are subsumed by the island paradigm. This body of theory has been used as a powerful tool for the analysis of species richness on real islands and for isolated habitats considered as islands (reviewed in Simberloff 1974; Connor and McCoy 1979). Following Janzen's (1968) suggestion, a number of researchers have attempted to apply island biogeography theory, especially the species-area curve, S = cAz, to situations involving animal hosts as islands for parasites (Dritschilo et al. 1975), symbionts of unspecified trophic relationships (Abele 1976; Abele and Patton 1976; Uebelacker 1977), and plant hosts as islands for herbivorous insects (see Strong 1979 for review) and mites (Tepedino and Stanton 1976). Hosts may be regarded as islands at three levels of organization: (1) individuals; (2) populations; (3) species. The studies of Abele (1976), Abele and Patton (1976), Tepedino and Stanton (1976), and Uebelacker (1977) treat individual host organisms or colonies as islands. Seifert (1975), Ward and Lakhani (1977), Lawton (1978), Freeland (1979), and Raupp and Denno (1979) study species richness using host populations as islands. Strong (1974c) and Strong et al. (1977) use the acreage of crops (cacao and sugar cane, respectively) planted in various political units (countries, states, and colonies) as islands. The remaining studies treat the geographic ranges of the host species as islands. The establishment and maintenance of species diversity is one of the central
TL;DR: Using five measures of relative fitness and other data, selection pressures which drive the habitat selection process have been quantified and theory of habitat selection is empirically extended to consider the more general condition of heterogeneous habitats.
Abstract: The gall-producing aphid Pemphigus betae is a plant parasite which colonizes the leaves of narrowleaf cottonwood, Populus angustifolia Habitat quality and aphid fitness can be quantified easily, and one may census not only those colonizers which succeed in reproduction but also those which die attempting colonization Using five measures of relative fitness and other data, selection pressures which drive the habitat selection process have been quantified (1) Colonizing stem mothers have evolved the ability to discriminate and select only the best habitats for colonization (2) As competitor density within a habitat increases, average fitness declines Stem mothers settle in habitats of varying quality such that as habitat quality increases the density of competitors increases (3) An individual leaf is a highly heterogeneous habitat Position within the habitat has a predictable and pronounced effect on fitness Due to position, the best habitats produce individuals with the highest and lowest fitness
TL;DR: It is suggested that the occurrence of butterfly species which deposit eggs in clusters is more common than the literature indicates and that data on egg deposition patterns in natural populations of nymphalids in North America, in particular for Phyciodes, Chlosyne, Euphydryas, and Nymphalis species, would support this conclusion.
Abstract: Egg clustering is found in certain butterfly groups such as nymphalids, pierids, and acraeids, but rarely in papilionids, satyrids, danaiids, riodinids, and hesperiids. I suggest that the occurrence of butterfly species which deposit eggs in clusters is more common than the literature indicates and that data on egg deposition patterns in natural populations of nymphalids in North America, in particular for Phyciodes, Chlosyne, Euphydryas, and Nymphalis species, would support this conclusion. Egg deposition patterns are a response to the structural and ecological characteristics of the larval host plants. The advantages of egg-clustering appear to be related to aposematic coloration in butterflies (eggs, larvae, and adults), although a particular stage in the life cycle of a butterfly that lays eggs singly may be aposematically colored.
TL;DR: The modified maximum principle is depicted as a discounted function of expected fitness, where the discount is a function of the variance in fitness between behaviors, and the function under no uncertainty reverts to the simpler mean maxim.
Abstract: I have shown how a maximum principle for evolutionary processes based solely upon mean fitness of behaviors may, under certain circumstances, be misrepresentative and misleading in the analysis of biological systems. Many of the misrepresentations can be corrected by including variance of fitness associated with environmental uncertainty and variability in the structure of the maximum principle. There are many ways of incorporating this variance into a mathematical model. I have chosen to depict the modified maximum principle as a discounted function of expected fitness, where the discount is a function of the variance in fitness between behaviors. Such a function under no uncertainty reverts to the simpler mean maxim. Including uncertainty generally results in evolutionary strategies consisting of sets of diverse behaviors whose resulting fitness show negative covariance. Under certainty organisms can specialize, showing only single types of behavior. Supporting evidence has been gathered from different ...
TL;DR: The gamma diversity patterns of California vegetation including the total flora and its various life-form and taxonomic subdivisions can be largely explained in the statistical sense by climatic and topographic variables, and thus serve as a reasonable test of several of the proposed environmental hypotheses of diversity regulation.
Abstract: The gamma diversity patterns of California vegetation including the total flora and its various life-form and taxonomic subdivisions can be largely explained in the statistical sense by climatic and topographic variables, and thus serve as a reasonable test of several of the proposed environmental hypotheses of diversity regulation. The actual relationship between environment and diversity patterns is more complex than ordinarily supposed. Topographic heterogeneity has a strong effect on patterns of the total flora and most subdivisions. Weather variables account for the bulk of the diversity patterns in the models used, but the results are unexpected in that favorableness indices (mean weather variables) are generally more important than indices of seasonality or irregularity. The results obtained in this study together with similar observations in other situations suggest that favorableness hypotheses deserve more theoretical and empirical attention.
TL;DR: It is proposed that the female-like plumage worn by some male birds in their first potential breeding season has evolved to facilitate breeding when 1-yr old through the deception of older males.
Abstract: We propose that the female-like plumage worn by some male birds in their first potential breeding season has evolved to facilitate breeding when 1-yr old through the deception of older males. By mimicking females 1-yr-old males exploit the tendency of old males not to attack females and, thus, are able to enter better quality habitats. Once subadults settle in such habitats, they hold territories in them by site dominance. An hypothesis invoking deception seems necessary because the cryptic hypothesis, which suggests that subadults are hiding from predators, cannot explain why subadult males of sexually dichromatic species always resemble females in every way by which they differ from older males. If the cryptic hypothesis were sufficient, subadult males should resemble juveniles in species for which the juvenile plumage is more cryptic than that of breeding females. Female mimicry should evolve (1) when competition for breeding resources is rigged against 1-yr-old males, (2) when obtaining a mate depends...
TL;DR: It is suggested that the model presented here be used as a heuristic device to stimulate and guide detailed, quantitative studies of the dynamics of cooperative and noncooperative behavior in animal groups.
Abstract: Put in one sentence, the central message of this article is: There are theoretical reasons and empirical evidence to suggest that the performance of cooperative and noncooperative behavior by animals in many situations is best explained by considering both fitness benefits and the contributions of other group members to cooperative behavior. The formal model presented here indicates one way in which fitness benefits and the actions of other group members can be combined to yield predictions about the performance of cooperative and noncooperative behavior. This model is contrasted with the population genetics models applying to cooperative and noncooperative behavior. These latter models predict only the presence or absence of cooperative and noncooperative behavior from fitness considerations; they do not predict the actual, quantifiable amount of each kind of behavior, and they do not recognize the possibility that the amount of a behavior may be adjusted in a manner contingent upon the behavior of other...
TL;DR: In this article, the authors examined the stability and community structure in mathematical models of competitive communities with "neutral models" and calculated stability, average alpha, and alpha variance of observed communities compared with those of analogous randomly constructed communities.
Abstract: Lyapunov stability and community structure in mathematical models of competitive communities are examined with "neutral models." The calculated stability, average alpha, and alpha variance of observed communities are compared with those of analogous randomly constructed communities. Calculated stability decreases as the number of species increases in observed communities. However, observed communities are generally more stable than randomly constructed communities with the same number of species. This greater stability of observed communities is partly due to the low values of both the mean and variance of their alpha distributions. Randomization of consumer resource utilization rates almost always increased the mean but not the variance of the calculated consumer similarities. In comparison to randomly constructed communities, the lower similarities and greater stability of the observed communities suggest that competitive processes are important in shaping real communities.
TL;DR: This paper attempts to extend the work of Levine (1976) in the context of the basic summary provided by Vandermeer and Boucher (1978), thereby providing a theoretical framework for the concept of "indirect mutualism" and suggesting that this form of interaction is likely to be extremely important in community organization, at least at some trophic levels.
Abstract: Although some of the most spectacular interspecific interactions in nature are obviously mutualistic, relatively little research, empirical or theoretical, has been aimed at understanding this basic and perhaps prevalent form of interaction (Risch and Boucher 1976). In this paper I attempt o extend the work of Levine (1976) in the context of the basic summary provided by Vandermeer and Boucher (1978), thereby providing a theoretical framework for the concept of \"indirect mutualism\" (S. James and D. Boucher, in prep.) or \"niche enhancement\" (Dodson 1970). I furthermore suggest that this form of interaction is likely to be extremely important in community organization, at least at some trophic levels. Vandermeer and Boucher (1978), using simple linear analysis, have categorized the basic forms of mutualistic interactions. In a fashion similar to Gause and Witt's analysis of competition (Gause and Witt 1935), Vandermeer and Boucher presented eight qualitatively distinct outcomes, predicted by a simple two-species system of differential equations. Their eight cases are summarized in table 1. Two features of their esults are of particular importance here, first hat the interactions may be stable or unstable, and second that mutualists may be facultatively mutual or obligately mutual. Note that the local stability of the system (the row headings as they appear in the first col. of table 1) has very little to do with the biological outcome in the usual sense. That is, in the case of interspecific competition, ifa local analysis indicates tability coexistence of the two species is automatically insured. However, no such correspondence exists with mutualism, as can be seen in table 1. Biological coexistence is possible when the system is mathematically unstable. That mutualists can be either obligate or facultative isobvious, and theoretically crucial since biological outcomes depend on this dicotomy (see table 1). The phenomenon of obligate mutualism has been modeled by assuming a negative carrying capacity (Vandermeer and Boucher 1978), an approach continued here. Levine (1976) extended the popular consumer esource quations of MacArthur (1968, 1972) to include interactions between resources. Levine's equations are shown diagramatically infigure 1. Levine noted that for a wide variety of parameter values the two consumers are \"effectively\" mutualistically associated with one another, as long as the resources are competitively associated. This effectively positive relationship shere called indirect mutualism, and is generated in an intuitively obvious fashion. If a resource required by a consumer is maintained at a relatively low biomass through competition with another resource, any factor
TL;DR: A more qualitative modeling approach derived from economics is proposed to take explicitly into account the degree of resource substitutability for predators requiring balanced diets, which yields hypotheses that differ markedly from those derived under the assumptions of perfect resource substitution.
Abstract: Foraging for complementary resources is widespread in nature. The conventional approaches to optimal foraging are inadequate to describe this phenomenon, since they are predicated on perfect substitution among alternative foods in terms of energy yield per unit time. A more qualitative modeling approach derived from economics is proposed to take explicitly into account the degree of resource substitutability for predators requiring balanced diets. The model yields hypotheses that differ markedly from those derived under the assumptions of perfect resource substitution. First, prey species cannot be ranked (ordinally or cardinally) in terms of their "value" to predators, since the benefits derived from consumption of one prey type depend on the quantities of complementary resources consumed. Second, prey selectivity is affected by the relative abundance of any potential food source. Third, when food resources are complementary predator fitness is greater on mixed diets than on single species diets.
TL;DR: It is speculated that the evolution of ecological versatility at the expense of competitiveness leads to coadaptation of the fauna throughout the archipelago for the level of diffuse interspecific competition characteristic of the largest islands.
Abstract: The paper is directed to interpreting the set of data displayed in figure 1, which shows that the number of bird species in two widespread West Indian habitats reaches a ceiling (saturation) that holds over a wide range of island species numbers. Four hypotheses are evaluated in an effort to understand the result: (1) Island communities are as tightly packed as they can be without breaching the limits of similarity; (2) island bird species are habitat specialists; (3) island species display an upper limit of tolerance of interspecific competition; (4) island species respond to reduced competition by broadening habitat spectra and other forms of ecological release. West Indian guilds are found to be less tightly packed than their mainland counterparts, eliminating hypothesis 1 in the sense of absolute limits to packing, but not foreclosing the possibility that island species are competitively excluded at lower packing levels than mainland species. The number of species per island increases in tight correla...
TL;DR: This thesis provides evidence that alarm signals act as "pursuit deterrent" signals and is related to that of Smythe (1970), who proposed that the display of the white rump patch in various herbivorous mammals "invited pursuit" from a predator.
Abstract: With few exceptions (Perrins 1968; Smythe 1970) it has always been expressly or tacitly assumed that the function of \"alarm\" signals was to warn conspecifics of the presence of a predator. To some it has also seemed self-evident that the signaler thereby decreased its chances of survival (Brereton 1959; Maynard Smith 1965; Hirth and McCullogh 1977). It has been the evolutionary implications of that second assumption that have been responsible for most of the interest in this topic. If it could be demonstrated that such behavior was not only altruistic but that it operated among social groups that were not kin groups it would provide evidence for \"group selection,\" the Achilles' heel of the theory of population self-regulation (Wynne-Edwards 1962). Opponents of these ideas can invoke the rule of parsimony and argue that this kind of behavior is not altruistic but self-serving (Smythe 1970; Charnov and Krebs 1975). However, we do not believe that these authors have been as parsimonious as they might have been. Our observations on the \"alarm\" signal of the eastern swamphen (Porphyrio prophyrio L.) indicate to us that this signal is intended for the predator, not conspecifics. Our thesis is related to that of Smythe (1970), who proposed that the display of the white rump patch in various herbivorous mammals \"invited pursuit\" from a predator. Our interpretation f how a predator is likely to respond to an alarm signal is, however, different. We provide evidence that alarm signals act as \"pursuit deterrent\" signals.
TL;DR: In this article, the authors used a graphical approach to predict the optimal time spent by a predator to consume a given portion of a target item while some material still remains, to search for more rewarding portions of another item.
Abstract: Many predators do not consume their prey whole. If these foragers feed on progressively less valuable portions of each prey item, they might maximize their net rate of energy intake by leaving an item while some material still remains, to search for more rewarding portions of another item. Using a graphical approach, I predict that, in general, increasing prey availability or value and decreasing search and handling costs should decrease the optimal time spent feeding on each item. For a given prey type similar predictions can be made concerning the optimal proportion of each item eaten. That a lower proportion of each item should be eaten when prey densities are higher was confirmed by a laboratory experiment. Acknowledging that many foragers do not consume their prey whole requires a modification of the concepts of a prey item and its value. Prey items should be defined by the forager, not by their spatial boundaries as perceived by an ecologist. An item's value should be its maximum possible net energy...
TL;DR: It is hypothesized that parthenogenesis is adaptive for habitats with unpredictable cycles of disturbance and resource abundance and is supported by a comparison of large and small lakes, which shows that a smaller proportion of zooplankton is parthenogenetic in great lakes.
Abstract: A model is developed to predict the minimum population density required (critical density) for successful sexual reproduction in animals. The model predicts that: (1) Hermaphroditic reproduction requires only half the critical density of bisexual reproduction; (2) a linear increase in mortality results in an exponential increase in critical density; and (3) increasing the time interval between clutches reduces critical density but makes a population more sensitive to external mortality. The probability of encountering a mate is dependent on the distance at which a male can detect a female. If predators can also detect females, there is an optimum conspicuousness for a female, which is dependent on the densities of males and of predators. A female should be very conspicuous only when both predators and males are rare. In an application of the model to zooplankton species, the smallest zooplankters are predicted to require relatively high densities for sexual reproduction. These animals reproduce with cycli...
TL;DR: Neither population size fluctuations (bottlenecks) nor the random processes and normative selection of mutation-selection balance theory can account for the differential correlation of group I and group II enzymes with the "organismic" and "environmental" group of niche descriptors.
Abstract: Protein polymorphism in 44 species of decapod Crustacea was assayed by electrophoresis. An average of 26 loci per species and 24 individuals per locus were surveyed. Each species (together with an additional seven species for which electrophoretic data were already available) was ranked on each of 10 niche descriptors: adult size, post-larval vagility, trophic level, trophic generalism, number of species per genus, decapod species diversity, latitude, productivity, euryhalinity and littorality; in addition each was ranked on two composite measures, Levinton's "index of opportunism" and a measure of trophic resource instability. Niche descriptors were intercorrelated over species and found to fall into three groups, an "organismic" group, an "environmental" group, and a third consisting of productivity and latitude. Maximum heterozygosities for 10 enzymes were intercorrelated. Though correlations were weak, the enzymes fell into two groups corresponding approximately to the groups I (central metabolic enzy...
TL;DR: Escape-behavior diversity is positively correlated with percent of tails broken at single-species sites and is also correlated with three estimates of predator abundance, although not significantly, which fit another hypothesis, namely, that lizards become wary after being attacked by a predator.
Abstract: Predators which form search images or learn to predict prey escape behavior should be hindered by diverse prey morphologies and escape behaviors. Thus, among conspecific prey populations, escape tactic diversities should vary positively with predation pressure and escape tactics should diverge among similar sympatric species that share predators. Escape behaviors were quantified for whiptail lizards at sites with only one species (Cnemidophorus tigris) scattered over the western North American deserts and for an assemblage of five species of sympatric whiptails in southwestern Texas. Relative predation pressure was estimated by frequency of broken tails. Several factors confound use of this index; however, percent broken tails is correlated with actual density of potential whiptail predators seen at sites. Escape-behavior diversity is positively correlated with percent of tails broken at single-species sites and is also correlated with three estimates of predator abundance, although not significantly. The...
TL;DR: It is argued that the relative importance of the three foraging costs is closely correlated with prey size, a comparatively simple variable to measure.
Abstract: The optimization approach to foraging is, by definition, concerned with the benefits and costs associated with feeding. The benefits to a predator are relatively easy to determine; they can, for example, be measured in terms of the weight or energy content of the food received. Measurement of costs however, has proven more difficult. Recently Werner and Hall (1974), followed by Kislalioglu and Gibson (1976), Stein (1977), and Elmer and Hughes (1978), have estimated costs as the time spent handling prey. Charles-Dominique and Hladik (1971) have calculated foraging costs from the energetic costs of movement while Griffiths (1980) measured costs by their effect on the predator's growth rate. This information has permitted some testing of the predictions made by the theory. However many of the predictions depend on the relative importance of the components of the overall foraging cost. For example, MacArthur and Pianka (1966) identify search and pursuit components of foraging and show that pursuing predators should be more selective than searchers. Here I classify predators according to their estimated search, pursuit, and subduing costs. While it has been recognized that predators are size selective (e.g., Brooks and Dodson 1965; Griffiths 1975), little attention has been paid to the relative sizes of predator and prey. I argue that the relative importance of the three foraging costs is closely correlated with prey size, a comparatively simple variable to measure. Finally, it is suggested that many of the distinctions employed by optimal foraging theorists can be accommodated within the cost framework.